A class-D amplifier, also sometimes called a switching amplifier, is a device for amplifying signals in which all of the power devices are operated as binary switches that are either fully on or fully off. Thus, the amplifier does not attempt to create the output directly, but rather makes a series of transitions between a defined set of discrete levels such that the time-average of the amplifier output is equal to the desired output. Most commonly, the defined set of discrete levels consists of two levels, and the binary switches are power transistors, for example MOSFETs or bipolar transistors.
The time-average output is typically determined by connecting a filter to the output of the amplifier, and desirable features may be achieved by including an inductor as part of the filter. In particular, if the output of a class-D amplifier passes through a low pass inductor-capacitor (LC) filter before reaching the intended load, the efficiency of the power delivered to the load is significantly higher than what is obtained from a conventional non-class-D amplifier configuration.
The improved efficiency is due in part to the fact that power dissipated in an amplifier itself, rather than the load, is largely the voltage drop in the output elements of the amplifier times the current delivered across that voltage drop (power equals voltage times current). However, in a class-D amplifier there is in theory no voltage difference between the currently active discrete level and the output, so that the power dissipated in the element used to connect to the currently active level is zero. Also, the elements that facilitate the output connection to any currently inactive levels conduct no current, so again the power dissipation in such elements is also zero. A class-D amplifier is thus theoretically 100% efficient.
In practice, a class-D amplifier is actually typically 90-95% efficient, clue to small but finite resistances in the switching elements, as the transistors are not perfect switches. By contrast, however, linear AB-class amplifiers always have both flowing current and voltage drops across the power devices, and thus lower efficiency.
FIG. 1 shows such a typical prior art class-D amplifier 100 having two discrete levels. Transistors M1 and M2 may form a complementary pair although this is not required; transistor M1 and its associated control input is used to connect the output to a first discrete Level One, while transistor M2 and its associated control input is used to connect the output to a second discrete Level Two. When transistor M1 is active, the amplifier output is at Level One; since there is no voltage drop across transistor M1, it dissipates no power. At this time, transistor M2 conducts no current, and thus also dissipates no power.
When the transistors M1 and M2 switch, the situation reverses. Now when transistor M2 is active, the amplifier output is at Level Two, and there is no voltage drop across transistor M2, and thus no power dissipation. Transistor M1 conducts no current now, and thus also dissipates no power. Thus, both possible output states result in no power dissipation by the amplifier elements.
Class-D amplifier 100 will execute a series of transitions between Level One and Level Two. As it does so, the average value of the amplifier output is created on the load connection by an LC filter. The average value will be a voltage in between Level One and Level Two, in proportion to the relative time spent at each level. For example, if the amplifier spends equal time at each level, the load connection will be at a voltage halfway between the Level One and Level Two voltages, while if the amplifier spends three times as much time at Level Two as at Level One, the load connection will be at a voltage that is three-quarters of the way between the levels.
The general expression of the load voltage is thus:Vload=V1+(V2−V1)*DutyCyclewhere V1 and V2 are the voltages of Level One and Level Two respectively and DutyCycle is the duty cycle of the amplifier, i.e., the fraction of the total time that the amplifier spends at Level Two. (One of skill in the art will appreciate that similar considerations apply where there are mere than two discrete levels, but the mathematics are more complex.)
Thus, by controlling the duty cycle of a class-D amplifier, the voltage on the load may be controlled, without dissipating any power in the elements thereof. One way to create a control signal that may be applied to the switching elements with the appropriate duty cycle is by a pulse width modulator (PWM). A PWM modifies its own output width such that a desired modulation is present in the duty cycle; when connected to a class-D amplifier, the PWM duty cycle will appear as a signal present in the voltage on the load. For example, in an audio application (a common use of class-D amplifiers) the PWM duty cycle encodes the audio signal. Which then appears on the load, for example, loud speakers.
In addition, class-D amplifiers are also compact, can switch states quickly, can be made at relatively low cost, and can deliver significant power, for example to power speakers in audio applications. These factors account for their commercial desirability.
However, the filtered output is not exactly proportional to the duty cycle of the PWM input signal. There may be different delays from the control signal to the switching elements, finite rise and fall times that are necessary to switch between the discrete output levels, and other non-idealities in the amplifier that result in an imperfect translation from the duty cycle input to the voltage on the load. This imperfect translation is distortion. Distortion can also vary depending upon the load, temperature and/or other operating parameters.
Thus, for example, a load such as a speaker may receive a signal that contains distortion in the audio output. In some cases, even, distortion of 0.1%, or one part in a thousand, may be audible to a listener.
For these reasons, a simple and inexpensive way of lessening the amount of distortion that occurs in a class-D amplifier would be useful.